In a method of manufacturing multiple transmission units with individually selectable gear ratios that vary from unit to unit, a gear pair having a desired transmission ratio is selected from a multiplicity of interchangeable gear pairs that includes both spiroplan gears and conventional crown gear and pinion combinations. The selected gear pair is then installed into a housing having first and second bearing sections configured to support bearings in which the shafts of the selected gear pair are rotatably mounted. The bearing sections are configured such that, for every gear pair that may be selected from among the multiplicity of gear pairs, the bearing sections support the first and second shafts in such a position and orientation that teeth of the gears engage with each other and a longitudinal axis of one of the gears of the gear pair is offset by a fixed distance from a longitudinal axis of the other. The fixed distance is maintained at a constant value which does not vary with or depend on the tooth configuration of the gear pair that is selected. A modular transmission system is thus provided, which can achieve a wide range of gear ratios, by an appropriate selection of interchangeable gear pairs, without need of modification of the basic components of the apparatus.